IWC Annual Report 2003 - Institut für Wasserchemie und chemische ...

Eh, V
6
−1.0 −0.5 0.0 0.5 1.0 1.5
1.1.3 Electrochemical Cleanup of Water Treatment Residues
Funding: BayFORREST F179
During the production of drinking water roughly 123.000 t (dry matter) of waterwork
residues acrue each year. Roughly 3/4 of these sludges are disposed and only 1/4 are
reused in other environments. In order to reduce the flux and to avoid accumulation of
enviromentally relevant contaminants, heavy metals and organic contaminants must
not exceed given concentrations.
Cr(OH)2+
Cr(OH)3
Cr(OH)4−
Cr+3
Cr3(OH)4+5
0 2 4 6 8 10 12 14
pH
CrO4−2
CrO4−3
CrOH+2
HCrO4−
Based on the idea of electrochemical soil reclamation,
the purpose of this project was a feasibility study to elaborate
the chemical, mineralogical, and technical conditions
for an electrochemical removal of heavy metal ions and
organic contaminants from waterworks residues. A potential
treatment process should be effective in terms of
flux reduction, treatment time and treatment costs. Of
course, the principal chemical and mineralogical characteristics
of the sludge, which render the sludge useful for
reuse in e.g. sewage treatment plants, must not be altered.
Electrokinetic treatment techniques are generally based
on electrophoresis, electroosmosis and electrochemical processes
at the electrodes. They usually involve a current
source, often filter materials, ion selective membranes, or
specific adsorber materials and sometimes electrolyte solutions
for a better control of the geochemical conditions
in the treated materials. Successful applications include
treatment of contaminated soils, contaminated wood, or
sewage sludge.
In contrast to these, the waterworks residues present a matrix which is rather sensitive
to changes of the pH/Eh conditions. Gibbsite, which is one of the major constituents
of the residue is stable only in the neutral pH range and dissolves at higher
and lower pH values as they occur at the electrodes. Even moderate treatment conditions
and pH-controlling electrolytes led to a quantitative dilution of the sludge. Once
dissolution takes place, the major constituents of the sludge are transported to the
corresponding electrodes and removed from the system. Afterwards the a reformation
of sludge is impossible.
Investigations also showed, that the heavy metal ions rather seem to be integrated
into the Gibbsite flakes than adsorbed to the surface of the sludge. Therefore it seems
impossible to remove the heavy metal ions without a destruction of the matrix.
Given the results of the experiments in the laboratory scale an application of electrochemical
methods to remove heavy metal ions from waterworks residues does not
seem feasible.
(D. Spangenberg)